Phase 3 (#1075): Migrate loop.py to run(gremlin)

[xbrianh]

Part of #1075. Depends on #1091 (Phase 2).

What

Same mechanical change as Phase 2 (run(gremlin), replace state.X with gremlin.state.X), but Loop has the most complex state interactions — it rebinds artifacts, records fields, calls record_bail(), and manages scope across iterations — so it gets its own phase.

Verify every state.* call in loop.py has a clear mapping through gremlin.state.*. Remove the State import.

Files

• gremlins/stages/loop.py